Bake Taxtures to one Image
It is possible. I have the similar case as yours. I've got a model, which consists of multiple separate parts. Each part has its own single material and own textures.


First join the meshes together with Ctrl+J.


Next create a new UV Map (press the '+' button in the UV Maps panel of the Mesh Data header and add a new image in the UV Editor window.


Unwrap your mesh with U-->Unwrap.


Go to Render header, set the bake mode to Textures and hit the BAKE button. It will create one big color texture map from the all textures you've apllied.




EDIT:
Here's a case, where object has one material and multiple textures applied to each face of the cube (via Unwrap-->Project from View(Bounds)). Every face has its own UV Map slot.


I then marked seams for the cube, unwraped it and created new UV Map for it (called 'all'). Then baked the Textures. And here's the result:


Hope it'll help you.